I'm trying to figure out how STP(802.1d) would react in the case of a new root bridge being added to a stable topology. The books talk about direct/indirect failues, but not really additions/new links. -An example: This network is convernged and the blocking port is the X by switch C. (A is the root)
/ \ / \ B-----X-C
-Lets say we were going to add a new root switch to switch A:
/ \ / \ B-----X-C
-So now switch Z is the root... What would be the downtime here? I am unclear on what ports would have to go through the Learning and Listening stages..if any.
-My Thoughts.. 1) Switch A recives a better BPDU so it gives up its root status to switch Z and forwards the better BPDU down to C and B 2) Switch B and C see the different BPDU and replace their old BPDU with the new one. -Would anything have to go through the Listening and Learning states? Thank You!
Good point. Let's try to think. SO first port which will go through listening/learning state is the one Between switches A and Z once it will be enabled. Then new BPDus from Z will start to flow.
Switch A will do calculation and select root port to be A-Z one no additional transituion needed. It then will start sending new BPDUs out of ports Ab and AC. Again no transition as those ports were forwarding before.
Switches B and C will receive new superiour BPDUs on root ports - will do calculations and leave those ports as root - again no transition between sates. Interesting happening when these BPDUs will be sent on ports BC and CB. So these both ports being non-root will receive superior BPDUs. I guess this should immidiately bring those in learning states. AFAIK Better BPDU on a non-root port = listening/learning/forwarding transition. During that they will again decide which side to block.
SO imho there will be two transitions 1) port AZ, 2) Later ports BC and CB.
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